High frequency modulated radio signal transmitter



y 19, 1942 I E. c. CORK ETAL 2,283,793

HIGH FREQUENCY MODULATED RADIO SIGNAL TRANSMITTER Filed.Feb 9, 1940 2Sheets-Sheet l CONDENSER MICROPHONE CONDENSER MICROPHONE CONDENSER IMICROPHONE INVENTOR EDWARD CECIL cok k JOgP/l L405 PAWSEY m ATTORNEY May19, 1942. E. c. coRK ET AL HIGH FREQUENCY MODULATED RADIO SIGNALTRANSMITTER Filed Feb. 9, 1940 2 Sheets-Sheet 2 CONDENSER MICROPHONECONDENSER MICROPHONE lNV EDWARD CEC ENTOR S /L CORK wag/l LADE PAWSE YATTORNEY Patented May 19, W42

HIGH FREQUENCY MODULATED RADIO SIGNAL TRANSMITTER Edward Cecil Cork,Ealing, London, and Joseph Lade Pawscy, Ickenham, England, assignors toElectric 8; Musical Industries Limited, Hayes, Middlesex, England, acompany of Great Britain Application February 9, 1940, Serial No.318,044

. In Great Britain June 7, 1939 Claims.

The present invention relates to frequency modulated radio signaltransmitters in which modulation of the output of the transmitter iseffected by operation of a transmitting device as a capacity microphone,reed or like device, of which the capacity varies in response to thesignals to be transmitted, and has particular but not exclusivereference to transmitters operating at the so-called ultra-short wavefrequencies, above about 50 m0. sec.

In order to obtain adequate output it isdesir- -able to use in atransmitter of the kind set forth an oscillator including a balancedpair or pairs of electron discharge valves. In this case the frequencycontrolling circuit, usually the anode circuit, of the oscillator, is abalanced circuit and the transmitter device is required to operate toproduce balanced capacity variations in the control circuit. Thus, incases whe're the transmitter device is connected directly in thefrequency control circuit both plates of. the device must, in mostcases, be subjected to high frequency and high tension voltages, whichis a disadvantage.

The object of the present invention is to provide an arrangement inwhich an unbalanced conence by way of example to the accompanyingdrawings of which the various figures show circuit diagrams of suitableoscillator arrangements for use in transmitters according to theinvention. Like reference numerals in difierent figures of the drawingsindicate like parts.

In the drawings Figs. 1 to 3, 3a and 4, inclusive, each illustrate awave generator of the balanced type, a modulating means of theunbalanced type with circuits and means for coupling the modu latingmeans to the balanced generator without upsetting the balance thereof.In Fig. 1, the generator is of the push-pull tuned circuit type with themicrophone coupled thereto by a transformer.

In Fig. 2, the tuned circuits of the generator.

comprise lines shunted by tuning straps and the modulator is coupled tothe generator by concentric lines. In Figs. 3, 3a and 4, the generatorsare in general as in Fig. 2. In the latter figures, however, themodulating means is coupled by concentric lines to one side ofthegenerating circuits and the effect thereof is counteracted by additionalmeans connected to the other side of the generating circuit.

Figure 1 of the drawings shows a simple form of the invention. :In thiscase the arrangement includes a pair of valves i connected in push-pullrelation, the valves having atuned grid circuit 2 including theinductance. capacity combination 3 and an output circuit 5 including thesecondary winding of transformer 5 to the mid-point of which thepositive pole of the high tension source 6 is connected in the usualway. The cathode be modulated by actuation of said microphone inaccordance with the signals to be transmitted. Preferably said tunedcircuit is a balanced line circuit and is excited by a pair of valvesconnected in push-pull relation. The microphone or other signalresponsive device is preferably connected in the tuned circuit by meansof a concentric line arrangement which is such that balanced capacityvariations are developed in the circuit includes suitable biasingarrangements for valves I, these arrangements being indicatedconventionally at 2a and 2bof the grid circuit.

The output circuit 4 may be tuned by the stray capacity of the circuitindicated by the condenser 5, shown dotted and the inductance of thesecondary winding of transformer 5 and the regenercarrier wave signalsare produced. The output of the transmitter is connected to the outputcircuit 4 in any suitable manner.

The arrangement illustrated in Figure 1 is not suitable for ultra shortwave operation, for example at frequencies above about 50 mc./sec., asin-this case the stray capacities of the circuits render satisfactoryoperation very difflcult. It is thus preferred to adopt, for ultra shortwave working, arrangements such as those shown in Figures 2, 3 and 4.

Thus, referring to Figure 2 the microphone is shown connected to aconcentric line So surrounded by a further cylinder 5b a quarter of awave length long extending from the end of the line So remote frommicrophone I towards the microphone and earthed at the end remote fromthe microphone and connected to the outer con-' ductor of line So at theend nearest the microphone. The conductors of the line 50. are connectedthrough blocking condensers 8 and 9 respectively one to each of a pairof anode conductors I and II associated one with each of the valves Iand tuned by the shorting bar I2 to which the high tension source 6 forthe anodes of valves I is connected.

The cylinder b serves two purposes. In the first place, it substantiallyeliminates unbalance in the output circuit between the plates of thetubes, and in the second place it substantially removes radio frequencycurrents from the surface of line 5a between the condenser microphoneand the point where line So is connected to the cylinder 5b. These twofunctions may be explained best separately. The preservation of balancemay most easily be shown by considering the outer surface of 5a and theinner surface of 5b as forming a quarter wave tank shorted at one end.It is well known that such a tank presents an extremely high impedance,the high impedance in this case occurring between ground and the pointwhere condenser 8 is connected to line So.

The impedance between the ground and the point where condenser 9 isconnected to the inner conductor of 5a is the sum of the impedancebetween the conductors of 5a and the very high impedance between theouter conductor of 5a and ground. Hence the effective impedance betweencondensers 8 and 9 is substantially solely the impedance between innerand outer conductors of 5a and the system is balanced because there isno admittance to ground in the circuit between condensers 8 and 9 ofappreciable magnitude.

On the other hand, to explain why the cylinder 5b substantiallyeliminates currents from the outer surface of 50, it is only necessaryto note that since the currents flowing on the inner and outerconductors of the quarter wave tank formed by the outer surface of 5a.and the inner surface of 5b, respectively, are necessarily substantiallyequal, the current flowing along the outside of 511 from left to rightis substantially neutralized at the point of contact with 5b by theequal and opposite current flowing from the contact point to the insideof 511. Thus of the current originally assumed to be flowing along theoutside of So on the left of this point of contact, there issubstantially nothing remaining to the right hand side of the point ofcontact.

Thus the cylinder 5b may be considered either as a by-pass path of theeffectively zero impedance for currents flowing along 50 or it may beconsidered as a neutralizing path so energized as to feed into the lineSo an equal and opposite current to that flowing along 5a. In any case.the currents on the outer surface of 5a are substantially eliminated andthis result is important because if any current were to flow on thisoutside surface the capacity between the line So and other objects wouldenter into the determination of frequency of oscillation, and this is,of course, highly undesirable.

For ultra short wave operation, the grid circult preferably includes thelecher wire arrangement 3a rather than the capacity inductancecombination 3 shown in Figure 1.

A further arrangement for ultra-short wave operation is shown in Figure3. In this case unbalanced to balanced working from the microphone I isobtained by the use of the concentric line I5 across one end of whichthe microphone I is connected and the other end of which extends insidethe tubular anode conductor I0 connected to the upper valve I as shownin the drawing. A cross connection is taken from the inner conductor ofline I5 through the D. C. blocking condenser 8 to some point on theoutside of anode conductor I I associated with the lower valve I asshown in the drawing. Also the control grid and anodes of the valves Imay be cross-connected through condensers I3 and I4 as shown, the leadsto these condensers passing through suitable apertures or slots inconductors III and II. The line I5 tends to cause unbalance and loss ofpower, but the balance is restored by the provision of a similarconcentric feeder I6 in association with the lower valve I. The feederI6 can be'used to afford a power output feeder of the transmitter bymeans of the adjustable cross connection between the inner conductor offeeder I6 and the outside of conductor I!) through the blockingcondenser 9. The-lines I5 and I6 are preferably interrupted as shown atI8, the gap being bridged by condenser I9 which serves in conjunctionwith the condensers 8 and 9 to prevent high tension voltage from source6 from being applied to either electrode of microphone I.

In Figure 3 the feeder I5 is shown as being tapped down the anode tunedconductor I0. This arrangement is adopted in order that the lossesintroduced by the line I5 will have less effect, but the use of thearrangement is not restricted to the case in which the feeder is tappeddown the conductor I0. In the arrangement shown in Figures 2 and 3 thefrequency of the transmitter is readily adjustable by sliding theshorting bar I2 along conductors I0 and II.

If desired, in order to improve the stability of the transmitter theanode of the valves I may be connected to tappings on the conductors IIIand II instead-of to the ends of the conductors as shown.

The line I5 is preferably between a quarter and a half wave length long,such that the end of the line I5 remote from the microphone presents anopen circuit.

Figure 3a shows a modification of the arrangement shown in Figure 3. Inthis case the lines I5 and I6 extend parallel to the conductors I0 andII, the inner conductors of lines I5 and I6 extending into theconductors I0 and II and being supported in the associated outerconductors and in conductors I0 and II' by means of insulating discs2'5, which give a low loss arrangement.

The shorting bar I2 is adjustable to provide a coarse frequency controland a fine control of frequency is afforded by providing slidable rods26 which are supported in sleeves on the outside of conductors I0 andII. Rods 26 can be actuated through a suitable device such as a rack orpinion to provide a ready means for close adjustment of the frequency.

Capacity coupling between conductors l and H and the outer conductors oflines and I6 respectively is afforded by arranging an insulating sheetbetween the tubes Ill and II and the metal chassis diagrammaticallyrepresented at 21, on which the arrangement is mounted, the outerconductors of lines l5 and 56 being in direct contact with the chassis.The chassis is preferably suitably recessed under conductors l0 and H toaccommodate the insulating sheet;

Variable condenser 28 may be connected to either line ID or II asdesired to balance the capacities of lines and II to earth.

A further possible arrangement according to the invention is shown inFigure 4. In this case the transformation of the microphone output froman unbalanced to a balanced condition is effected by means of a halfwave length concentric conductor 26 which is connected between two openwire sections 26 and 22 each approximately half a wave length long whichare connected to the anodes of the valves l. The high tension source 6is connected to the said anodes through the choke 23 which are connectedto voltage nodes 24 on the conductors 2| and 22. The conductors ofconcentric lines l5 and it are connected one to each end of theconcentric line 20 and the inner conductors thereof are connectedrespectively through blocking condensers 8 and 9 to the adjacentconductors 2| and 22.

We claim:

1. In a frequency modulation system an oscillation generator including apair of electron discharge devices each having input and outputelectrodes, oscillatory circuits connected with said input and outputelectrodes including lines symmetrically coupled with said outputelectrodes and balanced with respect to ground, an unsymmetricalmodulating reactance, and means for symmetrical modulating capacity, andmeans for coupling said unsymmetrical modulating capacity to saidbalanced circuits including a tubular conductor having a lengthapproximateLv equal to M4, a concentric line including inner and outermembers within said tubular conductor each member being connected tosaid modulatin capacity at one end and coupled to said balanced circuitsat the other end, and means connecting the outer member of theconcentric line to said tubular conductor to prevent transmission ofcurrents from the tuned circuits to the modulating capacity along theouter surface of the outer member of said concentric line.

3. In a frequency modulation system an oscillation generator including.a pair of electron discharge devices having electrodes connected in ahigh frequency tuned oscillatory circuit includeach of said tubes inpush-pull relation, said circuit being balanced with respect to ground,and means for modulating the frequency of the oscillations generated bymeans of an unbalanced condenser microphone without upsetting saidbalance of said tuned circuit'including, a condenser microphone, aconcentric line having its inner and outer members connected to-theelements of said condenser microphone at one end, a tubular member, aconnection between a point on the outer member of said concentric lineand said tubular member, said point being spaced from the end of saidtubular member a distance approximately equal to M4, and a couplingbetween the members of said concentric line and said first named lines.

4. In a frequency modulation system an oscil lation generator including,a pair of electron discharge devices having electrodes connected in ahigh frequency tuned oscillatory circuit including lines connectingcorresponding electrodes in each of said tubes in push-pull relation,said circuit being balanced with respect to ground, and means formodulating the frequency of the oscillations generated by means of anunbalanced condenser microphone without upsetting said balance of saidtuned circuit including, a condenser microphone, a concentric linehaving its inner and outer members connected to the elements of saidcondenser microphone at oneend and coupled to at least one of the linesof said oscillatory circuit at the other end. and a second concentricline coupled at least to the other of said lines of said oscillatorycircuit to restore the balance of said balcuit including linesconnecting corresponding electrodes in each of said tubes in push-pullrelation, said lines being balanced with respect to ground, and meansfor modulating the frequency of the oscillations generated by means ofan unbalanced condenser microphone without upsetting said balance ofsaid tuned circuit including,

a condenser microphone, a concentric line having its inner and outermembers connected to the elements of said condenser microphone at oneend and coupled to one of the lines of said oscilcondenser microphoneand to provide an output circuit.

6. In a frequency modulation system an oscillation generator including,a pair of electron discharge devices having electrodes connected in highfrequency tuned oscillatory circuits including conductors connectingcorresponding electrodes in each of said tubes in push-pull re-' edtothe elements of said condenser microphone at one end, means coupling theother end -of the members of said concentric line to one side of saidoscillatory circuit. a second concentric ing lines connectingcorresponding electrodes in 7'5 line coupled to the other side ofsaidoscillatorycircuit to provide an output circuit for said system, and ameans connecting the outer members of said concentric lines together andto ground at a point intermediate the ends thereof.

7. In a modulation system, a pair of electron discharge tubes havingelectrodes coupled in a balanced oscillation generator system includinga frequency determining tank circuit, a signal controlled unbalancedreactance element, a concentric line coupling said reactance element tosaid oscillation generator tank circuit for modulating the oscillationsgenerated in accordance with signals, a radio frequency voltage beingimpressed from the frequency determining oscillation generator tank uponthe signal controlled reactance element by way of said concentric lineover the surface of the outer member of which generated current mayreach the reactance element, and

' means for maintaining balance in said oscillation generator byeliminating generated radio frequency currents from the outer surface ofsaid line between a point on said line and said reactance elementincluding a conductor connected to the outer member of said line at saidpoint between said element and said tank circuit, and means to supply tosaid line at said point a current substantially equal and opposite tothe generated current flowing at said point on said line toward saidreactance element including a coupling between said conductor and saidtank circult.

ated current flowing at said point on said cable toward said reactance.

9. In a wave length modulation system a pair 'of electron dischargedevices each having a 8. In a balanced oscillator having two tubes Iwith lines connecting corresponding electrodes thereof in a push-pulltank circuit, and frequency modulating means including an unbalancedreactance variable in accordance with signals coupled to the said linesby a concentric cable over the outer member of which generated voltagesfrom said first lines may reach said signal controlled reactance, meansfor maintaining balance in said oscilaltor and for eliminating generatedradio frequency currents from the surface of the outer member of saidcable between a point on said cable and said reactance including aconductor the length of which is substantially v4 connected to the outermember of said cable at a point between said reactance and said tankcircuit and cooperating with said outer member to supply to said cableat said point a current substantially equal and opposite to thegenerplurality of electrodes including a cathode connected in a highfrequency oscillatory circuit including a tank circuit coupled tocorresponding electrodes in said devices and balancedwith respect tosaid cathodes. an unbalanced reactance variable at signal frequency, andmeans for coupling said variable reactance to said tank circuitincluding, a concentric line having its inner and outer conductorscoupled at one end to said variable reactance and at the other end tospaced points on said tank circuit, and means for restoring the balanceof said tank circuit upset by the coupling thereto of said variablereactance by said concentric line including, an additional conductorconnected to the outer conductor of said concentric line at a pointintermediate its ends to form with said outer conductor a resonant lineof a length equal to ,/4.

10. In a signalling system, a balanced oscillator including two tubeshaving anode electrodes and cathode electrodes with lines coupling saidanodes in a push-pull tank circuit balanced with respect to saidcathodes, and means for modulating the frequency of the generatedoscillations in accordance with signals including a variable condenser,and a concentric cable coupling said variable condenser to said lines,and means for restoring the balance of said circuit upset by couplingsaid condenser thereto and for preventing generated currents fromreaching said condenser over the surface of the outer member of saidconcentric cable including, a cylindrical conductor the length of whichis ,/4 surrounding said concentric cable, and a low impedance connectionbetween the end of said cylindrical conductor and a point on the outermember of said concentric cable between said element and said tankcircuit, said cylindrical conductor form ing with said outer member ofsaid cable a resonant circuit the length of which is i/4 to supply atsaid point a current substantially equal and opposite to the generated.current flowing at said point on said line toward said reactanceelement.

EDWARD CECIL CORK. JOSEPH LADE PAWSEY.

